System, Device and Method for Providing Call Forwarding in Dual Subscription Mode

1. A method, comprising: providing interworking between a first wireless telecommunication network and a second wireless telecommunication network, the first wireless telecommunication network and the second wireless telecommunication network being of different network types, wherein one of the first wireless telecommunication network and the second telecommunication network is an Internet protocol based network and the other wireless telecommunication network is a circuit-switched network; registering a dual-mode mobile terminal to one of the first wireless telecommunication network and the second wireless telecommunication network; forwarding a session request received by the first wireless telecommunication network and directed to a terminal unregistered from the first wireless telecommunication network to the second wireless telecommunication network; and checking whether the session request directed to the terminal is a request that can be successfully routed to the second wireless telecommunication network, wherein the forwarding is continued only when the checking determines that the session request can be successfully routed to the second wireless telecommunication network.

2. The method according to claim 1 , wherein the first wireless telecommunication network is an Internet protocol based network, and the second wireless telecommunication network is a circuit-switched network.

3. The method according to claim 2 , wherein the first wireless telecommunication network comprises an Internet protocol multimedia subsystem network.

4. The method according to claim 1 , wherein a same identifier is used in both the first wireless telecommunication network and the second wireless telecommunication network, the same identifier being configured to identify a called terminal.

5. The method according to claim 1 , wherein the dual-mode mobile terminal is informed after every location update on its registration in the second wireless telecommunication network.

6. The method according to claim 1 , further comprising: informing the first wireless telecommunication network of a registration of the dual-mode mobile terminal to the second wireless telecommunication network, wherein the first wireless telecommunication network is configured to use the information as a basis for a decision whether or not to route the session request to the second wireless telecommunication network.

7. The method according to claim 1 , further comprising: sending a message/information to the first wireless telecommunication network from the second wireless telecommunication network after a location update in the second wireless telecommunication network, wherein the first wireless telecommunication network is configured to record a time of the location update.

8. The method according to claim 1 , further comprising: sending a message/information to the first wireless telecommunication network when the dual-mode mobile terminal is switched off, the message/information indicating a switching off and unregistering of the first wireless telecommunication network from the second wireless telecommunication network, wherein the first wireless telecommunication network is configured to record an unregistered state.

9. The method according to claim 1 , further comprising: sending a message/information to the first wireless telecommunication network that the dual-mode mobile terminal will unregister from the second wireless telecommunication network, wherein the first wireless telecommunication network is configured to record an unregistered state.

10. The method according to claim 1 , wherein information about registering the dual-mode mobile terminal in the first wireless telecommunication network is continuously available in a server.

11. The method according to claim 1 , further comprising: checking whether the dual-mode mobile terminal is registered in the second wireless telecommunication network when the dual-mode mobile terminal is not registered in the first wireless telecommunication network.

12. The method according to claim 11 , further comprising: utilizing information gathered from the dual-mode mobile terminal for checking whether the dual-mode mobile terminal is registered in the second wireless telecommunication network.

13. The method according to claim 1 , wherein the forwarding comprises detecting whether the dual-mode mobile terminal is unregistered from the second wireless telecommunication network, and the method further comprises forwarding the session request to the second wireless telecommunication network when detecting that the dual-mode mobile terminal is registered to the second wireless telecommunication network.

14. The method according to claim 1 , further comprising: checking a time duration elapsed since a time point of registration of the dual-mode mobile terminal to the second wireless telecommunication network; and forwarding the session request to the second wireless telecommunication network when the time duration is shorter than a certain period, otherwise the session request is not forwarded.

15. The method according to claim 1 , wherein the first wireless telecommunication network is configured to provide a session forwarding service used when a subscriber is not reachable in the first wireless telecommunication network and the session request characteristics indicate that the session request can be successfully routed to the second wireless telecommunication network.

16. The method according to claim 15 , wherein the session forwarding service comprises a service for conditional voice call forwarding on a mobile not reachable in the first wireless telecommunication network.

17. The method according to claim 15 , wherein one of the components of the first wireless telecommunication network is configured to check whether a connection request to the dual-mode mobile terminal is a voice call request or a request for a packet-based connection or a data connection, and wherein the session forwarding service is continued only when detecting that the connection request represents the voice call request.

18. The method according to claim 1 , wherein the first wireless telecommunication network is configured to inspect, at least when the dual-mode mobile terminal is unregistered from the first wireless telecommunication network, session characteristics of an intended session for checking a possibility of successfully forwarding the session to the second wireless telecommunication network.

19. The method according to claim 1 , further comprising, when the first wireless telecommunication network receives a session set-up request comprising session or call characteristics, sending the session set-up request or at least a request comprising the session characteristics to an entity which checks, at least when a terminating terminal is unregistered from the first wireless telecommunication network, the session characteristics for deciding whether or not the one of the session set-up request and the request comprising the session characteristics can be routed to the second wireless telecommunication network.

20. The method according to claim 1 , wherein an entity of the first wireless telecommunication network checks whether a media gateway of the first wireless telecommunication network is configured to translate traffic content of the requested session to a form appropriate for forwarding the traffic content to the second wireless telecommunication network, and, when no proper conversion is available, the entity decides not to forward the session.

21. The method according to claim 1 , wherein, when the first wireless telecommunication network detects that the session cannot be successfully routed to the second wireless telecommunication network, the first wireless telecommunication network routes session to a video mail system.

22. The method according to claim 1 , wherein the first wireless telecommunication network is configured to route a session directed to an unregistered terminal directly to a media gateway, and, when the media gateway returns an error message because of lack of conversion of one of traffic and signaling of the session, an entity of the first wireless telecommunication network is addressed which decides how to further proceed with the requested session.

23. The method according to claim 22 , wherein the entity analyzes session characteristics and instructs the first wireless telecommunication network to forward, if possible, the session to a video mail system.

24. The method according to claim 1 , wherein an interface between a serving call session control function and an entity of the first wireless telecommunication network uses a session initiation protocol or is implemented as a service control point.

25. A system, comprising: a first wireless telecommunication network; a second wireless telecommunication network, the second wireless telecommunication network being a different network type from the first wireless telecommunication network; and a dual-mode mobile terminal configured to be registrable to at least one of the first wireless telecommunication network and the second wireless telecommunication network, wherein one of the first wireless telecommunication network and the second wireless telecommunication network is an Internet protocol based network and the other wireless telecommunication network is a circuit-switched network, wherein the system is configured to forward a session request received by the first wireless telecommunication network and directed to a terminal unregistered from the first wireless telecommunication network, to the second wireless telecommunication network, wherein the first wireless telecommunication network is configured to check whether the session request directed to the terminal is a request that can be successfully routed to the second wireless telecommunication network, and wherein the system is further configured to continue the forwarding only when detecting that the session request represents a request that can be successfully routed to the second wireless telecommunication network.

26. The system according to claim 25 , wherein the first wireless telecommunication network is an Internet protocol-based network, and the second wireless telecommunication network is a circuit-switched network.

27. The system of claim 26 , wherein the first wireless telecommunication network comprises an Internet protocol multimedia subsystem network.

28. The system according to claim 25 , wherein a same identifier is used in both the first wireless telecommunication network and the second wireless telecommunication network, the same identifier being configured to identify a called terminal.

29. The system according to claim 25 , wherein the system is further configured to inform the dual-mode mobile terminal after every location update on its registration in the second wireless telecommunication network.

30. The system according to claim 25 , wherein the system is further configured to inform the first wireless telecommunication network on the registration of the dual-mode mobile terminal to the second wireless telecommunication network, wherein the first wireless telecommunication network is configured to use the information as a basis for a decision whether or not to route a session request to the second wireless telecommunication network.

31. The system according to claim 25 , wherein after each successful location update in the second wireless telecommunication network, the dual-mode mobile terminal is further configured to send a message/information to the first wireless telecommunication network, wherein the first wireless telecommunication network is configured to record a time of the successful location update.

32. The system according to claim 25 , wherein the dual-mode mobile terminal is further configured to send, when the dual-mode mobile terminal is switched off, a message/information to the first wireless telecommunication network indicating a switching-off, and wherein the first wireless telecommunication network is configured to record an unregistered state.

33. The system according to claim 25 , wherein the dual-mode mobile terminal is further configured to send, when the dual-mode mobile terminal unregisters from the second wireless telecommunication network, a message/information to the first wireless telecommunication network indicating that the dual-mode mobile terminal will unregister from the second wireless telecommunication network, and wherein the first wireless telecommunication network is configured to record an unregistered state.

34. The system according to claim 25 , wherein the first wireless telecommunication network comprises a server configured to store information about registration of the dual-mode mobile terminal in the first wireless telecommunication network.

35. The system according to claim 25 , wherein the first wireless telecommunication network is configured to check, when the dual-mode mobile terminal is not registered in the first wireless telecommunication network, whether the dual-mode mobile terminal is registered in the second wireless telecommunication network.

36. The system according to claim 35 , wherein the system is further configured to check whether the dual-mode mobile terminal is registered in the second wireless telecommunication network using information gathered from the dual-mode mobile terminal.

37. The system according to claim 25 , wherein the system is further configured, when detecting that the dual-mode mobile terminal is unregistered from the second wireless telecommunication network, not to forward session request to the second wireless telecommunication network, and further configured, when detecting that the dual-mode mobile terminal is registered to the second wireless telecommunication network, to forward the session request is forwarded to the second wireless telecommunication network.

38. The system according to claim 25 , wherein the system is further configured to check a time duration elapsed since a time point of registration of the dual-mode mobile terminal to the second wireless telecommunication network, and to forward, when the time duration is shorter than a certain period, the session request to the second wireless telecommunication network, which otherwise is not forwarded.

39. The system according to claim 25 , wherein the first wireless telecommunication network is configured to provide a supplementary service for a conditional voice call forwarding on a mobile not reachable in the first wireless telecommunication network.

40. The system according to claim 25 , wherein the first wireless telecommunication network is configured to check whether a connection request to the dual-mode mobile terminal is a voice call request or the request configured to be successfully routed to the second wireless telecommunication network, and to continue call forwarding to the second wireless telecommunication network only when detecting that the connection request represents the voice call request.

41. The system according to claim 25 , wherein the first wireless telecommunication network is configured to provide a session forwarding service used when a subscriber is not reachable in the first wireless telecommunication network and session request characteristics indicate that the session request can be successfully routed to the second wireless telecommunication network.

42. The system according to claim 25 , further comprising: a session forwarding service comprising a service for conditional voice call forwarding on a mobile not reachable in the first wireless telecommunication network.

43. The system according to claim 25 , wherein the first wireless telecommunication network is configured to inspect, at least when the dual-mode mobile terminal is unregistered from the first wireless telecommunication network, session characteristics of an intended session for checking a possibility of successfully forwarding the session to the second wireless telecommunication network.

44. The system according to claim 25 , wherein, when the first wireless telecommunication network receives a session set-up request comprising session or call characteristics, the first wireless telecommunication network is configured to send one of the session set-up request and a request comprising the session characteristics to an entity which checks, at least when a terminating terminal is unregistered from the first wireless telecommunication network, the session characteristics for deciding whether or not the one of the session set-up request and the request comprising the session characteristics can be routed to the second wireless telecommunication network.

45. The system according to claim 25 , wherein an entity of the first wireless telecommunication network is configured to check whether a media gateway of the first wireless telecommunication network is configured to translate traffic content of the requested session to a form appropriate for forwarding the traffic content to the second wireless telecommunication network, and, when no proper conversion is available, the entity decides not to forward the session.

46. The system according to claim 25 , wherein, when the first wireless telecommunication network detects that the session cannot be successfully routed to the second wireless telecommunication network, the first wireless telecommunication network is configured to route the session to a video mail system.

47. The system according to claim 25 , wherein the first wireless telecommunication network is configured to route a session directed to an unregistered terminal directly to a media gateway, and, when the media gateway returns an error message because of lack of conversion of one of traffic and signaling of the session, an entity of the first wireless telecommunication network is addressed which decides how to further proceed with the requested session.

48. The system according to claim 47 , wherein the entity is configured to analyze session characteristics and to instruct the first wireless telecommunication network to forward, if possible, the session to a video mail system.

49. The system according to claim 25 , wherein an interface between a serving call session control function and an entity of the first wireless telecommunication network are configured to use a session initiation protocol or configured to implement a service control point.

50. A method, comprising: providing interworking between a first wireless telecommunication network and a second wireless telecommunication network, the second wireless telecommunication network being a different network from the first wireless telecommunication; registering a terminal to one of the first wireless telecommunication network and the second wireless telecommunication network; and forwarding a requested session, received by the first wireless telecommunication network and directed to a terminal unregistered from the first wireless telecommunication network, to the second wireless telecommunication network together with information indicating the session request was forwarded from the first wireless telecommunication network for providing interworking between the first wireless telecommunication network and the second wireless telecommunication network.

51. A method according to claim 50 , wherein at least one of the first wireless telecommunication network and the second wireless telecommunication network are configured to check information and to suppress the forwarding of the session request.

52. The method according to claim 51 , wherein the information comprises an identifier for identifying the unregistered terminal, the identifier being changed when forwarding the session request from one of the first wireless telecommunication network and the second wireless telecommunication network to the other of the first wireless telecommunication network and the second wireless telecommunication network.

53. A system, configured to: forward a session request received by a first wireless telecommunication network and directed to a terminal unregistered from the first wireless telecommunication network, to a second wireless telecommunication network; and forward, together with the session request, information indicating that the session request was forwarded from the first wireless telecommunication network for providing interworking between the first wireless telecommunication network to the second wireless telecommunication network, wherein the second wireless telecommunication network is a different network type from the first wireless telecommunication network.

54. The system according to claim 53 , wherein the first wireless telecommunication network and the second wireless telecommunication network are configured to check information and to suppress the forwarding of a session request.

55. The system according to claim 53 , wherein the information comprises an identifier for identifying the unregistered terminal, the identifier being changed when forwarding the session request from one of the first wireless telecommunication network and the second wireless telecommunication network to the other of the first wireless telecommunication network and the second wireless telecommunication network.